1. Field of the Invention
The present invention relates to actuators that move lenses back and forth in, for example, cameras and cellular phones, and in particular, relates to an electrostatic actuator that uses electrostatic force (coulomb force).
2. Description of the Related Art
A known electrostatic actuator is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2001-346385. FIG. 15 shows an overall structure of the electrostatic actuator shown in FIG. 1 in this document. FIG. 16 is a timing diagram showing driving signals that are applied to electrodes shown in FIG. 2 in this document.
As shown in FIG. 15, this electrostatic actuator includes a first stator 2a and a second stator 2b that are disposed so as to oppose each other with a predetermined distance therebetween and includes a slidable mover 3 that is disposed between the first stator 2a and the second stator 2b. 
The first stator 2a includes three chains of electrodes A, B, and C (a first electrode group) that are sequentially disposed in a predetermined direction. The second stator 2b includes a chain of an electrode D. The mover 3 includes electrode parts 3a that are disposed on a first surface of the mover 3 so as to correspond to a pitch of each of the electrodes A, B, and C provided in the first stator 2a and includes a flat electrode part 3d that is disposed on a second surface of the mover 3 and opposes the second stator 2b. The electrode parts 3a and 3d constitute a chain of an electrode E (a third electrode) and have the same electrical potential.
As shown in FIG. 16, when a voltage is applied to the electrode A provided in the first stator 2a so that an electrical potential of the electrode A becomes higher than an electrical potential of the electrode E provided in the mover 3, the mover 3 is attracted to the first stator 2a side by electrostatic force (coulomb force) that acts between the electrodes A and E. At this time, the mover 3 is acted upon by the electrode A so that the electrode A and the electrode parts 3a exactly overlap each other because a state in which the electrode A and the electrode parts 3a exactly overlap each other is most stable. Subsequently, when a voltage is applied to the electrode D provided in the second stator 2b in turn, the mover 3 is attracted to the second stator 2b side. Furthermore, when a voltage is applied to the electrode B provided in the second stator 2b in turn, the mover 3 is acted upon by the electrode B so that the electrode B and the electrode parts 3a exactly overlap each other on the basis of the same mechanism as in the case where a voltage is applied to the electrode A. When this series of operations is repeated, i.e., a cycle of processes for sequentially applying a voltage from a voltage source 6 through a switching circuit 5 to the electrodes A, D, B, D, C, and D is repeated (a voltage is alternately applied to one of the electrodes A to C provided in the first stator 2a and the electrode D provided in the second stator 2b while the electrode to which a voltage is applied in the first stator 2a is sequentially changed in the predetermined direction described above), the mover 3 is driven in the arrangement direction (the right side direction of the drawing) of the electrodes A, B, and C arranged in the first stator 2a from a macroscopic view while the mover 3 vibrates in the vertical direction from a microscopic view.
For example, in a case where the electrostatic actuator as described above is mounted in a camera so that the electrostatic actuator is used as an actuator that moves an automatic focusing lens, as much driving force as is needed to move the lens is required, and the movement speed, the response speed, and the like of the lens need to be increased. Thus, in such an electrostatic actuator, large electrostatic force (coulomb force) needs to be generated to gain large driving force.
In general, electrostatic force is proportional to the square of an applied voltage and areas of electrodes that oppose other electrodes, and is inversely proportional to the square of the size of gaps between electrodes. Thus, large electrostatic force can be gained by setting these factors to appropriate values.
However, in a case where an applied voltage is increased, there are limitations related to batteries that can be mounted in cameras, a withstandable voltage, and the like. Moreover, in a case where the size of gaps between electrodes is decreased, there are limitations due to the accuracy of manufacturing.
Moreover, in the known electrostatic actuator, the electrodes in the first stator 2a and the second stator 2b are disposed so that plane areas of these electrodes oppose plane areas of the electrode in the mover 3. Thus, the size of the plane area of each electrode in the first stator 2a, the second stator 2b, and the mover 3 needs to be increased to increase the area of each electrode opposing another electrode. However, in this arrangement, the size of the electrostatic actuator itself is increased. Moreover, the weight of the mover 3 is increased. In spite of the original intention, the movement speed, the response speed, and the like of the mover 3 are disadvantageously decreased.
Moreover, the automatic focusing lens actuator described above moves the lens from an initial waiting position to a target position (a focusing position). In such an automatic focusing lens actuator, it is preferable that the following two types of feed be available: rough feed that shortens the time required to move the lens by increasing the movement speed of the lens upon moving the lens from the waiting position and fine feed that accurately stops the lens at the target position by decreasing the movement speed of the lens when the lens is approaching the target position.
However, the known electrostatic actuator described above has a structure in which the movement speed of the lens is not variable, and rough feed and fine feed of the lens cannot be alternately selected. Thus, it is difficult to accurately stop the lens at the target position. Moreover, when an accurate stop position of the lens is required, the lens needs to be moved back and forth several times. Thus, focusing operation requires a long time.
Moreover, a problem due to the structure exists, such that it is difficult to keep an even movement speed of the lens.